The present invention relates to a polymerization inhibitor and a polymerization-inhibiting method for a vinyl compound.
The vinyl compound generally has easily polymerizable properties by light and heat, etc. Therefore, to prevent the polymerization of a vinyl compound, various kinds of polymerization inhibitors are used alone or in combination with other polymerization inhibitors in accordance with the surrounding conditions of the vinyl compound.
An example of a particularly highly polymerizable vinyl compound includes (meth)acrylic acid. Hereinafter, in this specification, (meth)acrylic acid indicates both methacrylic acid and acrylic acid.
Hitherto, as methods for producing (meth)acrylic acid, there have been frequently used, for example, a method for producing acrylic acid by a catalytic gas phase oxidation reaction of propylene and/or acrolein, and a method for producing methacrylic acid by a catalytic gas phase oxidation reaction of isobutylene and/or methacrolein, etc.
In these methods, gas produced from the catalytic gas phase oxidation reaction is cooled and collected by water to obtain an aqueous solution of (meth)acrylic acid containing such by-products as acetic acid and an aldehyde, etc. and, from this aqueous solution, (meth)acrylic acid is separated and purified by combining processes such as distillation and extraction by a solvent. The method for producing (meth)acrylic acid by the forementioned catalytic gas phase oxidation reaction involves a process to separate, enrich, and purify, etc. the acid by distillation. Also, as methods for producing (meth)acrylic acid other than the above-described, there have been known a method for producing acrylic acid by Reppe's method and a method for producing methacrylic acid by the acetone cyanhydrin method, etc. In these methods there has been involved a process to separate, enrich, and purify (meth)acrylic acid by distillation.
The (meth)acrylic acid is very easily polymerized by light and heat, etc. Therefore, the polymerizing properties of (meth)acrylic acid increases, especially, under high temperature conditions such as in the forementioned distillation process. Accordingly, when highly polymerizable (meth)acrylic acid like the forementioned is industrially produced, polymerization of these acids in a distillation process is a very important item in operation of the process. Further, to establish an effective polymerization-preventive art in a distillation process the acid under a high temperature is an essentially important item for operating the process continuously for a long period of time and under a stable condition. According to the knowledge of present inventors, the polymerization most easily takes place in a dehydrating process (a water-separating process) during the forementioned distillation course and this fact is a main factor which disturbs continuous operations of a production device including a distillation column.
When subjecting (meth)acrylic acid to operations such as separation, enriching, and purification using a distillation column, most of the places where a polymerization product is easily formed by a polymerization reaction of the acid are areas with scarce wetting by liquid such as a rear side of a tray, an inside of bubble cap, a rear side of a downcomer, a hollow on a column inside, etc. and a place of easily liquid-stagnating such as a metal fittings (bolt and nut, etc.) for putting-in a tray and a packing part, etc. The polymerization product is hardly soluble in (meth)acrylic acid, water, and usual organic solvents and, once formed in a column, it becomes a seed for polymerization and gradually accumulates. Finally, the column inside is clogged by it and a continuous operation of the production device becomes impossible. Further, it is very difficult to remove these accumulated polymerization products.
Conventionally, a method of carrying out distillation in the presence of a polymerization inhibitor has been adopted as a measure for preventing polymerization of (meth)acrylic acid in a distillation process when producing (meth)acrylic acid. As polymerization inhibitors of this kind, is hitherto known a polymerization inhibitor (A) composed of the following ingredients: at least one kind of compound selected from the group consisting of hydroquinone, methoquinone (p-methoxyphenol), cresol, phenol, tertiary-butylcatechol, diphenylamine, phenothiazine, and methylene blue, at least one kind of copper dithiocarbamate selected from the group consisting of copper dimethyldithiocarbamate, copper diethyldithiocarbamate, and copper dibutyldithiocarbamate, and molecular oxygen (for acrylic acid, refer to Japanese Official Patent Provisional Publication, showa 49-85016; for methacrylic acid, refer to Japanese Official Patent Gazette, showa 57-61015). Hitherto-known polymerization inhibitors of (meth)acrylic acid other than the above are a polymerization inhibitor (B-1) composed of a manganese salt such as manganese acetate etc., a polymerization inhibitor (B-2) composed of a manganese salt such as manganese acetate etc. with hydroquinone and/or methoquinone, a polymerization inhibitor (B-3) composed of a manganese salt such as manganese acetate etc. with molecular oxygen, and a polymerization inhibitor (B-4) composed of a manganese salt such as manganese acetate etc., hydroquinone and/or methoquinone, and molecular oxygen (for all of these, refer to Japanese Official Patent Provisional Publication, showa 51-98211).
However, the inventors examined polymerization-inhibiting effects of the above-mentioned known polymerization inhibitors of (A) and from (B-1) to (B-4), so that the inventors found that the polymerization inhibitors of (A) and from (B-1) to (B-4) have the following problems.
That is, the polymerization inhibitor (A) disclosed in the Japanese Official Patent Provisional Publication, showa 49-85016 and Japanese Official Patent Gazette, showa 57-61015, has low polymerization-inhibiting effects when polymerization of (meth)acrylic acid very easily occurs by containing water, acetic acid, and aldehyde compounds in the feed composition as in an azeotropic separation column (a water-separating column) of an aqueous (meth)acrylic acid solution in the distillation process, when (meth)acrylic acid is produced by a catalytic gas phase oxidation reaction. Due to the low inhibiting effect, there was a problem that, with generation of a popcorn polymer and a sticky polymer in the course of distillation, continuous operation in a long period of time of a production device including a distillation column became impossible. Also, since this polymerization inhibitor (A) contains a copper salt having a corrosive character, there is a problem that the distillation column easily corrodes during a continuous operation in a long period of time.
On the other hand, the polymerization inhibitors of from (B-1) to (B-4) disclosed in the Japanese Official Patent Provisional Publication, showa 51-98211, contains a manganese salt having a low corrosive character, so that there is no problem about corrosion of the distillation column. The polymerization inhibitors of from (B-1) to (B-4) have sufficient polymerization-inhibiting effects on the products from (meth)acrylic acid, but the polymerization-inhibiting effects is insufficient, similarly to the case of the polymerization-inhibitor (A), in a case where polymerization of (meth)acrylic acid very easily occurs as in an azeotropic separation column of an aqueous (meth)acrylic acid solution in the distillation process, when (meth)acrylic acid is produced by a catalytic gas phase oxidation reaction. Thus, there was a problem that, with generation of a popcorn polymer and a sticky polymer during distillation, continuous operation over a long period of time in a production device became impossible.
Also, there were problems similar to the forementioned in the conventional polymerization inhibitors and polymerization-inhibiting methods of a vinyl compound other than (meth)acrylic acid.